The present invention relates generally to web processing equipment and, more specifically, to methods and apparatus for steering the web as it travels through the processing equipment.
Web coating refers to the process of depositing one or more materials onto a thin heat-sensitive substrate, supplied in the form of a continuous, large-quantity roll (i.e., a xe2x80x9cwebxe2x80x9d). The web is unwound and passed through one or more stations in processing equipment, each of which deposits a material onto the web, generally under vacuum. After the web passes through all of the stations, it is rewound onto another roll and readied for further processing or shipment. One product typically manufactured by such a process is magnetic recording tape, where a polyester film is coated with ferromagnetic material.
Processing equipment used for web coating generally includes a series of web transport rollers. These rollers unwind, tension, steer, and wind the web as it travels through the processing equipment. Frictional forces at the interface between the web and a roller provide traction that causes the web to move through the equipment in the direction that the rollers establish.
FIG. 1 depicts a simplified schematic representation of an example of web processing equipment 100. Processing equipment 100 includes a source 102 of web material 104 in roll form. A pumping system 116 with a pumping port 118 helps maintain the proper pressure (e.g., vacuum) in the processing equipment 100. Also included are three material deposition stations 106, 108, 110, each of which can deposit a different material onto the web 104. As the web 104 exits the last material deposition station 110, it is rewound onto a take-up roll 112.
Generally included in each material deposition station 106, 108, 110 is a web transporting roller 114 and a steering section 200. Material is deposited on the web 104 as the web 104 travels about the web transporting roller 114. FIG. 2 shows the steering section 200 in detail. In the present example, the steering section 200 includes an entering roller 202 that receives the leading edge of the web 104. The web 104 is then received by a first guide roller 204 and a second guide roller 206. The guide rollers 204, 206 are parallel to each other. The web 104 then passes to an exit roller 208 en route to the web transporting roller 114 and then on to another material deposition station or the take-up roll 112.
One common problem with the aforementioned processing equipment 100 is a misalignment 210 between the entering roller 202 relative to the guide rollers 204, 206. An additional misalignment 212 between the exiting roller 208 and the guide rollers 204, 206 can also happen. When one or both of these conditions occur, the steering section 200 often cannot restore the proper alignment. A result of this condition is that wrinkles, or creases, or both, form in the web 104 as it travels through the processing equipment 100. The wrinkles or creases tend to accelerate degradation of the coated web product. Further, the wrinkles or creases can hamper the efficient operation of the processing equipment 100, causing downtime as an equipment operator must, for example, manually realign the web 104.
From the foregoing, it will be apparent that there is still a need for a way to control the travel of the web 104 through the processing equipment 100 in order to minimize or eliminate wrinkles, or creases, or both, with minimal or no operator intervention.
The present invention affords control over the degree to which a web floats over (i.e., avoids contact with) or tracks (i.e., contacts) a web transporting roller in processing equipment, such as coating machinery, typically when under vacuum. Adjusting the web tension, or web transporting roller configuration, or both, facilitates this control. A result is that web wrinkling or creasing induced by, for example, steering or other rollers, is reduced or eliminated. This leads to improved performance and decreased downtime of the processing equipment, and minimizes degradation of the web product.
In one embodiment, the invention provides a method for prescribing the operational web tension pressure in relation to the web outgassing pressure. Briefly, outgassing is the evolution of gas embedded in a liquid or solid. The gas is typically released when the liquid or solid is heated, or the surrounding pressure is reduced, or both. In one embodiment discussed herein, the web is placed under vacuum in the processing equipment, which causes the release of gas embedded in the web. By adjusting the web tension pressure so it is less than the web outgassing pressure, the web is caused to float over the web transporting roller. Conversely, setting the operational web tension pressure so it exceeds the web outgassing pressure will cause the web to track the web transporting roller. The choice of causing the web to float or track depends on, for example, the type of web material used and the nature of the processing performed.
One way of setting the operational web tension pressure is to compute the molecular density of the substance (typically water vapor) outgassing from the web. From this density value one can compute a target web tension pressure and set the operational web tension pressure accordingly.
The operational web tension pressure can also be controlled dynamically. During operation, a system according to the invention monitors the web outgassing pressure. The system then adjusts the operational web tension pressure to maintain the desired relationship between it and the web outgassing pressure. The adjustment can be performed in real time, automatically, and without operator intervention, typically using a closed-loop control system.
In another embodiment, the invention provides a method for prescribing the degree to which a web floats or tracks as it passes over one or more web transporting rollers by selecting a particular configuration of rollers for use in the processing equipment. The configuration is based in part on the average surface roughness of each web transporting roller. The average surface roughness required to achieve a desired degree of floating or tracking may be based at least in part on a computed molecular density of the substance outgassing from the web and a desired operational web tension pressure. Web transporting rollers having the appropriate average surface roughness corresponding to the molecular density and the desired operational web tension pressure are then selected for use in the equipment.
One version of the invention includes apparatus for adjusting the operational web tension pressure. In this version, sensors monitor the ambient pressure due to outgassing about the web transporting roller and the tension pressure of the web. A tensioning roller responds to the ambient pressure and adjusts the web tension pressure to maintain the desired relationship between the pressure values. A variation of this version includes a controller that accepts data from the pressure sensors and operates an actuator that, in turn, adjusts the tensioning roller accordingly. A further variation includes the logic necessary to compute molecular density and target web tension pressure and adjust the tensioning roller in relation to the latter.
Another embodiment of the invention provides an article of manufacture that includes a program storage medium having computer-readable program code for prescribing the degree to which a web floats about a web transporting roller. The code includes portions for monitoring web outgassing pressure during web transporting roller operation, and adjusting operational web tension pressure in relation to the monitored pressure. Additional code causes a computer to compute a web outgassing molecular density and target web tension pressure, and set the operational web tension pressure in relation to the latter. In another embodiment, a program storage medium tangibly embodies a program of instructions executable by a computer to perform the method steps for prescribing the degree to which a web floats about a web transporting roller.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating the principles of the invention by way of example only.